The present invention relates in general to preparing a solution by mixing through a filter, and more specifically to preparing a solution through a filter to remove residue from an integrated circuit.
Semiconductor technology is producing integrated circuits that include more transistors with smaller dimensions and more detailed geometric features. The accurate reproduction of such devices requires a highly planar wafer surface with low levels of particles and other residue. Hence, where accurate geometries are needed, most manufacturing processes include surface planarization steps.
For example, a chemical-mechanical polishing (CMP) step is often used to polish away a blanket tungsten layer to the remaining tungsten plugs or to planarize an interlayer dielectric (ILD) material that isolates one interconnect layer from another. The wafer surface is mechanically polished using a slurry comprised of alumina particles suspended in an aqueous solution having a pH of 1.8-3.8. However, the low pH of the aqueous solution produces a zeta potential in the alumina particles opposite to that of the wafer, which creates an attraction force that causes many of the alumina particles to adhere to the wafer surface. Such particles produce photolithographic defects and wafer contamination which can result in a failure of the integrated circuit.
The particles are removed with a cleaning solution whose pH is set to about 12.0 to alter the polarity of the zeta potential and weaken the attraction force between the wafer and the particle. Typically, an ammonium hydroxide solution having a 2% concentration by weight provides the correct pH for removing particles. Control over the concentration is important. Too low a concentration does not sufficiently raise the pH and therefore inhibits particle removal by inadequately altering the zeta potential of the particles. Too high a concentration results in a more caustic solution which can corrode wafer materials such as tungsten plugs which are used to fill interlayer via openings.
Ammonium hydroxide is commercially available at a 30% concentration by weight. Prior art processes produce a 2% concentration by manually diluting 30% ammonium hydroxide with de-ionized (DI) water in a bulk storage tank, where it is hand-mixed and stored until needed. However, hand-mixing and hand-pouring such a strongly basic solution presents a safety hazard to people handling the chemical and those working in the area of the storage tank. Moreover, ammonia gas is highly volatile. When ammonia evaporates from the solution, not only does the safety hazard increase, but also the concentration of the NH4OH in the cleaning solution drops significantly, lowering the pH of the diluted solution below acceptable levels.
Hence, there is a need for an apparatus and method of forming a homogeneous mixture of fluids, such as preferably a dilution of ammonium hydroxide, by mixing through a filter. More particularly, there is a need for an apparatus and method to remove residue from a semiconductor wafer which reduces evaporation and avoids the risk of exposing nearby persons to hazardous chemicals.